1. Field of the Invention
The present invention relates to materials with non-linear optical properties, especially although not exclusively Langmuir-Blodgett (L-B) films with such properties, and to optical devices utilising such materials and films.
2. Description of Related Art
A material is optically non-linear if its dielectric constant is a function of the applied electric fields and can be expressed by: EQU .chi..sub.0 +.chi..sub.1 E.sub.1 +.chi..sub.2 E.sub.1 E.sub.2 +.chi..sub.3 E.sub.1 E.sub.2 E.sub.3 + . . .
where .chi..sub.0, .chi..sub.1, .chi..sub.2 . . . are dielectric susceptibilities of the material, and E.sub.1, E.sub.2 . . . are the electric fields. The electric fields may be optically applied or may be electrically applied AC or DC fields, or any combination of these depending on the situation. Of the terms .chi..sub.2, .chi..sub.3 . . . which give rise to non-linearity, .chi..sub.2 is usually the largest, and is a function of the molecular hyperpolarisability .beta. of the molecules in the material and the orientation of those molecules. .beta. is zero if the molecules are centrosymmetric and .chi..sub.2 is zero if the arrangement of the molecules in the material is centrosymmetric, irrespective of the value of .beta.. Thus, in order to provide an optically non-linear material it is necessary to align molecules of high .beta. in a non-centrosymmetric configuration.
Known optically non-linear materials include potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP) and lithium niobate (LiNbO.sub.3). However certain organic molecules have larger values of .beta. and it is an object of the present invention to provide an improved optically non-linear material in the form of a multi-layer Langmuir Blodgett (L-B) film.
The Langmuir-Blodgett technique for the deposition of thin films involves passing and re-passing a substrate through a thin film of a suitable organic material lying on the surface of a polar liquid such as water, under carefully controlled conditions. An ordered monolayer of the organic material is deposited on the substrate at each pass of the substrate. Suitable organic materials are usually elongate non-centrosymmetric molecules with hydrophilic "heads" and hydrophobic hydrocarbon "tails". Unfortunately the most common mode of deposition (Y deposition) involves the deposition of successive monolayers in a head-to-head and tail-to-tail configuration, which is thus centrosymmetric. It has been found particularly difficult to deposit organic molecules of high hyperpolarisability (.beta.) in any configuration other than the unwanted head-to-head/tail-to-tail Y configuration.